Quantum State Manipulation of Atomically-Precise Gold Nanoclusters by Controlling Size & Structure

Atomically-precise gold nanoclusters have gained wide attentions due to their unique quantum state and related intriguing properties. These nanoclusters exhibit non
metallic (i.e., molecular) electronic structure unlike bulk or plasmonic nanoparticle of gold. The non-metallic electronic structure of a nanocluster is very sensitive to the size
and structure, even at the atomic level of difference. Recent progress in the atomically precise synthesis and crystallographic structure determination has provided chemists with the wide availability of nanoclusters with different size/structure for the exploration of novel functionalities. However, some fundamental questions are not clearly answered yet. For example, the critical role of the structure (e.g., core and/or surface) for the properties
still remain unclear. The transition from non-metallic to metallic state has not been identified at the atomic level of precision due to the difficulty in atomically-controlled
synthesis of a nanocluster near the transition size region (i.e., ~2 nm). The elucidation of the origin of surface plasmons also needs further investigation of the nascent state. In this thesis, I tackle the major issues of nanoscience by synthesis and structure determination of thiolate-protected gold nanoclusters. I first discuss oxidation-induced
size/structure transformation (OIST) of atomically-precise Au nanoclusters from one stable size to another (Chapter 2). The observed transformation from [Au23(SR)16]− to
Au28(SR)20 demonstrated the first case of novel synthetic procedure by oxidation as well as the atomic insights into the charge state control of a nanocluster. In Chapter 3 and 4, I discuss atomically-tailored core and/or surface structures of
Au nanoclusters, and the effect on the optical properties. I have controlled the crystallize phase of the core structure in an atomically-precise Au30(SR)18 for the first time by a
novel ligand-based strategy (Chapter 3). The ligand-based strategy realized the hexagonal-close packed (hcp) structure in the Au30(SR)18, unlike face-centered cubic (fcc) structure in previously reported Au30(SR’)18, and bulk or plasmonic NP of gold.
Interestingly, the controlled crystalline phase in the same sized NP led to totally different properties. This work has demonstrated a strategy for controlling nanocluster structure
(hcp vs. fcc) to tailor the optical properties without changing the size in atomic precision. I have also performed surface tailoring of Au nanoclusters to control the optical
properties of Au nanoclusters (Chapter 4). Au103S2(SR)41 and Au102(SR’)44 nanoclusters are protected by different ligands (i.e., 2-naphthalenethiolate and paramercaptobenzoic acid) and the two nanoclusters show the same Au79 core but different surface structure.
Steady-state spectroscopy revealed the same UV-Vis absorption profile of Au103 and Au102, but ultrafast spectroscopy identified different excited-state lifetimes (420 ps for Au103 vs. 3.5 ns for Au102). The correlation between spectroscopic and structural analyses has elucidated the critical role of core and/or surface structure of Au nanoclusters for their optical properties. The atomically-tailored optical properties discussed in Chapter 3
& 4 demonstrate a strategy for quantum state manipulation of non-metallic Au nanoclusters.
In Chapter 5−8, I discuss the atomically-precise identification of the transition from non-metallic to metallic state of Au nanoclusters and the nascent state of surface
plasmons. Chapter 5 describes the sharp transition from nonmetallic Au246(SR)80 to metallic Au279(SR)84 within merely 33 Au atoms, which goes against >50 years of
theoretical prediction of a smooth transition. I overcame the difficulties in controlling the atomic monodispersity by a ligand-based strategy and successfully synthesized Au279.
The “nascent” surface plasmon in Au279(SR)84 exhibited intriguing optical properties unseen in typical plasmons. Chapter 6 & 8 focus on the unique features of such a nascent surface plasmon near the transition size region. I have discovered anomalous electron dynamics in metallic Au333(SR)79 with unprecedented relaxation process, in addition to electron-phonon coupling and phonon-phonon coupling as seen in typical plasmonic Au nanoparticles (Chapter 6). The nascent state of surface plasmons is further explored through the synthesis and spectroscopic analysis of Au~300(TBBT)~84 and Au~310(pMBT)~80, resulted in the observation of intriguing electron dynamics (Chapter 8). Chapter 7 includes the discussion on the effect of alloying on the non-metallic to metallic transition through the case of Au130-xAgx nanoclusters with non-metallicity.
Overall, the conclusion derived from this dissertation has provided critical insights (e.g., structure and/or size) into the quantum state of Au nanoclusters at atomic
level. Further studies toward the direction will guide one to the strategy to engineer the functionality of nanomaterials by quantum state manipulation for the applications in
optoelectronic, catalysis, and biomedicine, as well as quantum computing.